1. Field of the Invention
The present invention relates to a wheel speed sensor for use in detecting wheel speeds of vehicles such as automobiles and motorcycles. Further, the present invention relates to a terminal to which an electric wire is connected in which a plurality of thin metallic wires are bundled together to constitute a conducting portion and a method for welding the electric wire to the terminal.
2. Description of the Related Art
In general, wheel speed sensors attached to wheels of a vehicle, respectively, to detect wheel speeds of a running vehicle are designed to detect the rotation of rotors which rotate together with axles coupled to the respective wheels. The rotors contain a magnetic material and magnetic sensors such as Hall ICs and MR elements are used to detect the rotation of the rotors.
However, in the event that the magnetic sensors such as Hall ICs are provided directly on the axle portions of the running vehicle, there is a risk that the sensors maybe damagedby flying stones, and moreover, a connecting portion between lead portions and electric wires is liable to be disconnected.
To cope with this problem,it is a conventional common practice in wheel speed sensors to mold a detection element such as a Hall IC by a synthetic resin together with a connecting portion with the cords to improve the durability thereof.
As an example of the resin-sealed wheel speed sensor, a wheel speed sensor disclosed in JP-A-2000-206130 is conventionally known.
The construction of this conventionally known wheel speed sensor will be briefly described with reference to FIG. 11.
A wheel speed sensor has a detection element 110 such as a Hall IC, electric wires 112 connected to the detection element, an accommodating portion 113 for accommodating the detection element 110 and a lid unit 114 for covering an upper face of the accommodating portion 113 in which the detection element 110 is accommodated.
The electric wires 112 are connected to terminals 111 of the detection element 110 by welding or clamping. Then, the detection element 110 to which the electric wires 112 are so connected is accommodated in the accommodating portion 113.
The lid unit 114 is attached to the upper face of the accommodating portion 113 in which the detection element 110 is accommodated.
Finally, an assembly into which the detection element 110, the electric wires 112, the accommodating portion 113 and the lid unit 114 are assembled is disposed within a mold (comprising a stationary mold half and a movable mold half) (not shown) so that the whole periphery of the assembly is sealed with a synthetic resin, whereby the wheel speed sensor is formed.
As has been described heretofore, in the conventional wheel speed sensor, the electric wires have to be connected to the terminals of the detection element by welding or clamping, the detection element to which the electric wires are connected has to be accommodated in the accommodating portion, and the lid unit has to be mounted on the accommodating portion prior to sealing with the resin, this causing a problem that a number of man hours are required to assemble the respective constituent component parts.
In addition, in this construction in which the detection element is accommodated in the accommodating portion for sealing with the resin after the electric wires have been connected to the terminals of the detection element, stress is applied to the terminals and the electric wires before they are accommodated in the accommodating portion, and this may trigger a risk that the connection between the detection element and the electric wires is disconnected.
In addition, in order to position an assembly to which the electric wires are connected within a mold, positioning projections are provided on an accommodating portion and a lid unit in such a manner as to project therefrom, and positioning portions are formed in the mold.
In resin molding the assembly, the assembly is placed in an opened mold (a stationary mold half) and is positioned by bringing the positioning projections into engagement with the positioning portions on the mold side, and thereafter, the assembly is clamped with the stationary mold half and a movable mold half and is them molded with a resin to form a housing.
In the wheel speed sensor disclosed in the aforesaid JP-A-2000-206130, of the positioning projections provided at a plurality of positions around the periphery of the assembly, firstly, the projections corresponding to the positioning portions on the stationary mold half are brought into engagement with the same positioning portions, and then the positioning projections corresponding to the positioning portions on the movable mold half are brought into engagement with the same positioning projections. Due to this, the posture and position of the assembly within the cavity tend to remain unstable until the assembly is clamped by the mold halves. Since the assembly is placed in the cavity while the electric wires are connected to the assembly with an L-shape, the assembly is liable to be twisted or floated within the cavity. When there is caused a deviation in position of the assembly within the cavity, it is not possible to form a uniform resin sealed portion, whereby the penetration of water or the like into the assembly from an exposed portion thereof is liable to happen.
In addition, in the event that a plurality of positioning portions are attempted to be provided on both the assembly and the mold (the stationary and movable mold halves), a production cost for the assemblage of the component parts is increased, and, time has to be spent in placing the assembly in the mold and therefore the assemblage of the component parts is deteriorated. Moreover, the mold and the wheel speed sensor itself have to be enlarged.
Next, a conventional method for welding an electric wire in which a plurality of thin metallic wires are bundled together to constitute a conducting portion (a signal wire) to another member will be described with reference to FIGS. 12 and 13.
FIG. 12 shows an electric wire 210 having a certain thickness being welded to a metallic terminal 211.
An electrode 212 for use in the welding is constituted by two electrodes; one of them is one electrode 212a which contacts a conducting portion 213 of the electric wire 210 and the other is the other electrode 212b which contacts the terminal 211. A recessed groove 214 is formed in the one electrode 212a for accommodating therein the conducting portion 213.
The conducting portion 213 is held in the recessed groove 214 formed in the one electrode 212a and is then pressed against the terminal 211 by the one electrode 212a. 
When welding, electric current is allowed to flow from the one electrode 212a toward the other electrode 212b through the conducting portion 213 and the terminal 211 (as shown by an arrow A in the figure). The conducting portion 213 and the terminal 211 are heated by the current for welding.
Thus, in a case where the electric wire 210 of a predetermined thickness is welded to the terminal 211, since the conducting portion 213 is accommodated in the recessed groove 214 in the electrode 212, the conducting portion 213 can be welded to the terminal 211 without getting discrete to the individual thin metallic wires when welding.
FIG. 13 shows an electric wire 215 of a diameter which is smaller than that of the electric wire shown in FIG. 12 being welded to a terminal 211. In addition, FIG. 13 shows that a conducting portion 216 gets discrete to individual thin electric wires 219.
In this case, too, one electrode 218a is disposed on a conducting portion 216 side in which a recessed groove 217 matching the conducting portion 216 in size is formed, while the other electrode 218b which is flat is disposed on a terminal 211 side.
Note that in the event that the diameter of the electric wire is small, the individual thin metallic wires 219, 219 . . . constituting the conducting portion 216 are also considerably thin. Thus, the conducting portion 216 constituted by the thin metallic wires 219 is originally easy to get discrete.
Furthermore, the depth of the recessed groove 217 formed in the one electrode 218a for accommodating the conducting portion 216 tends to be shallow.
Thus, it is difficult to align the shallow recessed groove 217 in the one electrode 218a with the thin conductor 216 so as to snugly accommodate the conductor 216 therein and press it against the terminal 211 for welding.
Consequently, in a case where the electric wire 215 having the thin conducting portion 216 is welded, it is preferable to use a flat electrode having no recessed groove 217 formed therein as the electrode 218a which is pressed against the conducting portion 216.
However, with the thin conducting portion 216 which is originally easy to get discrete, in the event that a flat electrode is attempted to be used for welding, there is a high possibility that the conducting portion gets discrete more easily.
In the event that the conducting portion gets discrete, as shown in FIG. 13, when attempting to weld the thin electric wire, a contact area of the individual discrete thin metallic wires with the electrode increases and this causes a problem that even if electric current is allowed to flow, generating heat becomes difficult.
Furthermore, even if welding can be attained, there is caused a problem that the overall strength of the conducting portion is remarkably deteriorated due to the influence of heat on the individual thin metallic wires.
On the other hand, in the event that welding is carried out with the conducting portion being in a discrete condition, and that a plurality of terminals are provided in close vicinity to each other, there is caused a risk that the discrete thin metallic wires may be brought into contact with the other terminals which are located nearby.
In addition, JP-B-4-45946 and JP-A-6-218552 describe methods for welding a conducting portion while preventing the conducting portion from getting discrete.
According to the method described in JP-B-45946, in welding an electric wire to a terminal or the like, since a restraining jig for restraining the conducting portion is used, the conducting portion can be prevented from getting discrete.
With this method, however, since the restraining jig has to be used, there is caused a problem that man hours are increased for attachment and detachment of the restraining jig at the time of welding.
Furthermore, according to the description in JP-A-6-218552, there is provided a welding method in which a terminal having a curved surface and electrodes having curved surfaces which match the curved surface of the terminal are used, and therefore, this method cannot be adopted in a case where welding is performed to a flat terminal.